Head pressure when walking after sitting for long periods

I'm wondering if anyone else may have my symptoms. About a year ago, I got out of the car and walked into a store. I started getting this terrible pressure starting at the base of my head that seemed to go up and over the top of my head to my face. I felt like my eyes were going to pop out of my head, and then it started to subside. I felt like I was going to faint, but didn't. Since then it has happened many times. Sometimes after sitting a long time at work or after riding in the car. It is never immediately on standing, but after I have walked from the parking lot into a store or down the hall at work. It lasts 1-2 minutes and then it is over. It almost feels like a seizure. I went a while and it didn't happen often. It is happening more again, but not as much pressure. I just stop and stand there, wondering if this time I will actually faint, and wait for it to go away. I am not having the extreme pressure anymore where my eyes feel that they are bugging out.
My PC ordered an MRI on my head since I am a breast cancer survivor, but nothing showed up.
I'm having some minor vertigo issues 3-4 times over the last year as well.
I'm 61 female, breast cancer survivor, TSH chemo 2008, Armour Thyroid for Hashimoto's thyroid and on BP medicine which is very stable.
I would appreciate hearing if anyone has experienced this.